Some optical devices include a light transmitter and/or a light receiver, collectively termed a light transceiver, that transmits and/or receives a light beam. The light transceiver may be pointed in a specific direction to select the direction of the light path along which the light beam that is transmitted and/or received. An example of a light transmitter is a laser rangefinder or illuminator, and an example of a light receiver is a light sensor such as a focal plane array. Some light transceivers may include both a light transmitter and a light receiver.
In each case of this type of optical device, the light transceiver is pointable in a selected direction. For the case where the light transceiver is a light transmitter, the outgoing light beam is pointed or steered in the selected direction. For the case where the light transceiver is a light receiver, the light receiver is pointed to select the angle of the incoming light beam.
The pointing or steering may be accomplished either by mounting the light transceiver on a gimbal or by keeping the orientation of the light transceiver fixed in space and altering the direction of the light path using an optics subsystem that employs mirrors, lenses and the like mounted on a gimbal but with the light transceiver mounted off the gimbal. The latter approach with the light transceiver mounted off the gimbal is preferred, because it usually results in a lower mass that must be pivoted and a lower total mass. Two-dimensional X-Y pivoting gimbals or roll-nod gimbals are widely used in seeker systems.
The optical device of this type is typically positioned behind a window that protects the optical device. The window must be sufficiently large that the light path may be pointed to the required maximum pointing angle and still pass through the transparent window. Because of the relatively large size of the gimbal, the gimbal must be well-spaced behind the interior surface of the window to provide mechanical clearances during the pivoting. The result is that the window must be relatively large to accommodate the gimbal and also allow the light path to be steered to the required maximum pointing angle.
The requirement for a large window has several undesirable effects. First, the larger the window, the more expensive it is to produce with the required high optical quality. The large window also adds a substantial amount of weight to the structure, an important concern for some types of applications such as light transceivers in lightweight unmanned aerial vehicles. For military applications where stealth is desirable, the larger window typically increases the radar and infrared signatures of the platform (e.g., the aircraft) on which the optical device is mounted.
For many of the applications of the optical devices of this type, the light transceiver must operate at two or more wavelengths. For example, a laser illuminator that transmits a laser beam may operate at a wavelength of 1.55 micrometers, and an associated infrared receiver may operate in the medium-wavelength infrared (MWIR) band of 3-5 micrometers or in the long-wavelength infrared (LWIR) band of 8-12 micrometers. It is desirable that any beam-steering device be optically functional at the required two or more wavelengths. Some possible steering approaches can in principle reduce the window size, but have so much chromatic aberration that they are satisfactory only for a single wavelength or a narrow wavelength band.
There is thus a need for an optical device with beam-steering capability that allows the size of the window to be reduced, and also permits operation at two or more wavelengths which may be widely separated in the optical spectrum. The present invention fulfills this need, and further provides related advantages.